The invention is directed to a dispersion compensating optical fiber and a transmission system including the same, and more particularly to a dispersion compensating optical fiber and transmission system in which the dispersion compensating fiber exhibits a negative dispersion and dispersion slope within the C-band (1525 nm to 1565 nm) to advantageously compensate for slope and dispersion in the transmission system.
High data rates are becoming needed for the telecommunications industry. Thus, the search for high-performance optical fibers designed for long distance, high bit rate telecommunications in Dense Wavelength Division Multiplexing (DWDM) systems has intensified. However, these high data rates have penalties associated with them. In particular, dispersion is a significant problem for such systems, particularly those employing large effective area fibers, such as certain Non-Zero Dispersion Shifted Fibers (NZDSF). More specifically, positive dispersion builds as a function of the length of the transmission fiber (e.g., a NZDSF). Dispersion Compensating (DC) fibers included in a cable or in a Dispersion Compensating Module (DCM) have been designed that compensate for such dispersion in such optical transmission systems. These DC fibers generally have negative slope and negative dispersion such that a short length of the DC fiber may be used to compensate for the positive dispersion and positive slope of the longer transmission fiber, for example a NZDSF. For C-band operation between about 1525 nm and 1565 nm, the bend performance, attenuation, and dispersion properties (total dispersion and/or dispersion slope) of the DC fiber are very important. This is particularly true in DC fibers that will be included in a wound spool of a DCM. In particular, having very low total dispersion is advantageous as it allows for compensation with less DC fiber length. Low slope is desirable to compensate for the slope of the transmission fiber in a short length.
Thus, there is a need for a DC fiber that exhibits low attenuation, low bend loss, low dispersion and dispersion slope and is particularly effective at compensating for the dispersion and/or slope of certain Non-Zero Dispersion Shifted Fibers (NZDSF) over the C-band.
The following definitions are in accordance with common usage in the art.
The refractive index profile is the relationship between refractive index and optical fiber radius.
A segmented core is one that has multiple segments, such as a first and a second segment (a central core and a moat, for example). Each core segment has a respective refractive index profile and maximum and minimum index therein.
The radii of the segments of the core are defined in terms of the beginning and end points of the segments of the refractive index profile or in terms of the midpoint of the segment in the case of a ring segment. FIG. 2 illustrates the definitions of radii used herein. The same definitions are used for FIGS. 3-6 as well. The radius R1 of the center core segment 22 is the length that extends from the DC fiber""s centerline (CL) to the point at which the profile crosses the relative refractive index zero as measured relative to the cladding 30. The outer radius R2 of the moat segment 24 extends from the centerline (CL) to the radius point at which the outer edge of the moat crosses the refractive index zero, as measured relative to the cladding 30. The radius R3 is measured to the half height width where xcex943% is half its maximum value of the ring segment 26. The radius R3 of segment 26 extends from the centerline (CL) to the midpoint 28 of a half-height line segment 27. The midpoint 28 is formed by bisecting the segment 27 between the two intersection points with the ring segment at the half height position of xcex943%. The radius R4 is measured from the centerline (CL) to the point where the outermost portion of the ring segment 26 meets the zero refractive index point, as measured relative to the cladding 30.
The effective area is defined as:
Aeff=2xcfx80(∫E2 r dr)2/(∫E4 r dr), where the integration limits are 0 to ∞, and E is the electric field associated with the propagated light as measured at 1549 nm.
The term, xcex94%, represents a relative measure of refractive index defined by the equation,
xcex94%=100 (nl2xe2x88x92nc2)/2nc2
xe2x80x83where nl is the maximum refractive index in the respective region i (e.g., 22, 24, 26), unless otherwise specified, and nc is the refractive index of the cladding (e.g., 30) unless otherwise specified.
The term alpha profile, xcex1-profile refers to a refractive index profile, expressed in terms of xcex94(b) %, where b is radius, which follows the equation,
xcex94(b)%=[xcex94(bo)(1-[|bxe2x88x92bo|/(b1xe2x88x92bo)]xcex1)]100
xe2x80x83where bo is the maximum point of the profile and b1 is the point at which xcex1(b)% is zero and b is in the range bixe2x89xa6bxe2x89xa6bf, where xcex94% is defined above, bi is the initial point of the xcex1-profile, bf is the final point of the xcex1-profile, and xcex1 is an exponent which is a real number. The initial and final points of the xcex1-profile are selected and entered into the computer model. As used herein, if an xcex1-profile is preceded by a step index profile, the beginning point of the xcex1-profile is the intersection of the xcex1-profile and the step profile. In the model, in order to bring about a smooth joining of the xcex1-profile with the profile of the adjacent profile segment, the equation is rewritten as;
xcex94(b)%=[xcex94(ba)+[xcex94(bo)xe2x88x92xcex94(ba)]{(1-[|bxe2x88x92bo|/(b1xe2x88x92bo)]xcex1}]100,
xe2x80x83where ba is the first point of the adjacent segment.
The pine array bend test is used to compare relative resistance of optical fibers to bending. To perform this test, attenuation loss is measured when the optical fiber is arranged such that no induced bending loss occurs. This optical fiber is then woven about the pin array and attenuation again measured. The loss induced by bending is the difference between the two attenuation measurements. The pin array is a set of ten cylindrical pins arranged in a single row and held in a fixed vertical position on a flat surface. The pin spacing is 5 mm, center to center. The pin diameter is 0.67 mm. The optical fiber is caused to pass on opposite sides of adjacent pins. During testing, the optical fiber is placed under a tension sufficient to make the waveguide conform to a portion of the periphery of the pins.
The DC fiber in accordance with the invention disclosed and described herein is particularly well suited to compensating for dispersion and dispersion slope of certain NZDSF in the C-band.
According to an embodiment of the invention, a DC fiber is provided having a refractive index profile selected to provide a particular set of properties (attributes) that make it suited for transmission systems designed to operate in the C-band wavelength window of between about 1525 nm and 1565 nm.
The DC fiber in accordance with the invention is particularly suitable for compensating for build up of dispersion and/or dispersion slope in an NZDSF having a kappa of about 50. Thus, the DC fiber may be coupled to a NZDSF to form a transmission system and is designed to compensate for the dispersion and/or slope (and most preferably both) of the NZDSF, preferably across the entire C-band. The transmission system including the DC fiber may also preferably include optical amplifiers, filters, Wavelength Division Multiplexing operation, and other conventional system components. Preferably, the DC fiber is wound onto a spool and included in a Dispersion Compensating (DC) module.
In accordance with an embodiment of the invention, the total dispersion defined herein as the measurable dispersion (total dispersion equals chromatic dispersion plus the waveguide dispersion plus profile dispersion) of a transmission system employing 100 km of a NZDSF transmission fiber and a suitable length of the present invention DC fiber advantageously results in a system which has less than +/xe2x88x9240 ps/nm residual dispersion; more preferably less than 35 ps/nm and most preferably less than 30 ps/nm over the entire C-band (between 1525 nm and 1565 nm). Fiber profiles have been designed in accordance with the invention that exhibit attenuation of less than 3 dB/km at 1549 nm. Moreover, the bend loss, as measured by a pin array test, is preferably less than 40 dB, more preferably less than 30 dB, and most preferably less than 25 dB. Thus, the DC fiber in accordance with the invention exhibits excellent bend loss and may be, therefore, advantageously wound onto, and used in, small diameter DCM""s to be utilized in transmission systems for compensating dispersion and dispersion slope of long lengths of NZDSF.
In a preferred embodiment of the DC fiber, a plurality of segments characterize the fiber and each of the segments of the core includes a refractive index profile. Together, these segments make up the refractive index profile of the fiber. At least one of the segments preferably has an xcex1-profile. Most preferably the fiber includes a core profile having a positive xcex941% central core segment, a negative xcex942% moat region, and a positive xcex943% ring segment. Preferably, the ring segment has a non-step index profile and its radius R3 is offset from the moat segment.
According to the present invention, the DC fiber has a segmented core having at least three segments. The refractive index profile of the segmented core is selected to provide a negative total dispersion and a negative dispersion slope at 1549 nm, and more preferably over the entire C-band from 1525 nm to 1565 nm. In accordance with the invention, the DC fiber exhibits a highly negative dispersion slope. In particular, the DC fiber""s dispersion slope is more negative than xe2x88x923.4 ps/nm2-km at 1549 nm, and the DC fiber includes a moat delta xcex942% that is more negative than xe2x88x920.4%.
In accordance with another embodiment of the invention, a DC fiber is provided having a dispersion slope is more negative than xe2x88x924.0 ps/nm2-km at 1549 nm.
Preferably, the present invention DC fiber has a total dispersion at 1549 nm more negative than xe2x88x92125 ps/nm-km. In a further embodiment of the invention, the total dispersion more negative than xe2x88x92165 ps/nm-km at 1549 nm. In further embodiments, the total dispersion is more negative than xe2x88x92200 ps/nm-km; and may be more negative than xe2x88x92250 ps/nm-km. Preferably, the total dispersion for the DC fiber at 1549 nm ranges between about xe2x88x92100 and xe2x88x92300 ps/nm-km. In certain embodiments, the total dispersion may range between about xe2x88x92165 and xe2x88x92270 ps/nm-km at 1549 nm. Most preferably, the total dispersion for the DC fiber ranges from xe2x88x92100 to xe2x88x92165 ps/nm-km.
In some embodiments, the DC fiber may exhibit an even more highly negative dispersion slope that is more negative than xe2x88x924.0 ps/nm2-km at 1549 nm. In the most negative slope embodiments, the dispersion slope may be more negative than xe2x88x924.5 ps/nm2-km; and even more negative than xe2x88x925.0 ps/nm2-km at 1549 nm. Preferably, the dispersion slope ranges between xe2x88x923.4 and xe2x88x926.3 ps/nm2-km at 1549 nm. In other embodiments, the dispersion slope may range between xe2x88x924.5 and xe2x88x926.0 ps/nm2-km at 1549 nm. In all embodiments, the dispersion slope of the DC fiber is preferably more negative than xe2x88x921.5 ps/nm2-km over the entire C-band from 1525 nm to 1565 nm.
The DC fiber preferably has a kappa value, defined as the total dispersion at 1549 nm divided by the dispersion slope at 1549 nm, of less than 60 nm; and more preferably less than 52 nm. Preferably, kappa is between 35 nm and 55 nm for all embodiments. In preferred embodiments, kappa ranges between 40 nm and 52 nm; and most preferably between 40 nm and 48 nm. Kappa for the DC fiber is preferably between 35 nm and 75 nm over the entire C-band range between 1525 nm to 1565 nm.
The DC fiber in accordance with the invention has a central core segment having a positive xcex941%, a moat segment adjoining the central core segment and having a negative xcex942%, and a ring segment surrounding the moat segment having a positive xcex943%, all as compared to cladding that is preferably pure silica. According to the invention, xcex941% of the central core segment is preferably greater than 1.7%. xcex942% of the moat segment is preferably more negative than xe2x88x920.4%; and more preferably more negative than xe2x88x920.65%. The xcex943% of the ring segment is preferably greater than 0.5%.
The DC fiber in accordance with embodiments of the invention preferably exhibits, in combination, a central core segment having a positive xcex941% greater than 1.5%, a moat segment adjoining the central core segment and having a negative xcex942% more negative than xe2x88x920.5%, and a ring segment surrounding the moat segment having a positive xe2x88x923% greater than 0.5%.
According to further embodiments, the DC fiber in accordance with the invention preferably exhibits, in combination, a central core segment having a positive xcex941% between 1.6% and 1.9%, a moat segment adjoining the central core segment having a negative xcex942% between xe2x88x920.6% and xe2x88x920.75%, and a ring segment adjoining the moat segment having a positive xcex943% between 0.65% and 1.2%.
The effective area of the DC fiber at 1549 nm in accordance with the invention is greater than 13 xcexcm2; more preferably greater than 15 xcexcm2; and most preferably greater than 17 xcexcm2.
In accordance with another embodiment of the invention, an optical transmission system is provided having a dispersion compensating optical fiber with a refractive index profile being selected to provide a dispersion slope more negative than xe2x88x923.4 ps/nm2-km at 1549 nm and wherein the fiber includes a xcex942% more negative than xe2x88x920.4%. Preferably also, the dispersion compensating fiber exhibits a total dispersion at 1549 nm more negative than about xe2x88x92125 ps/nm-km.
In accordance with yet another embodiment of the invention, an optical transmission system is provided having a dispersion compensating optical fiber with a refractive index profile selected to provide a dispersion slope more negative than xe2x88x924.0 ps/nm2-km at 1549 nm.
Further features and advantages of the invention will be set forth in the detailed description which follows, and will be readily apparent to those of ordinary skill in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings. It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate several embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.